1. Field of the Invention
The present invention relates to a method of producing a ceramic rotor having a shaft portion and a circular disk portion as a ceramic turbine wheel for a turbocharger, gas turbine, etc.
2. Description of the Prior Art
Various methods as slip casting, injection molding, press forming, cold isostatic pressing, etc. are known for producing ceramic rotors of the above described kind. Among those methods, injection molding is widely used for reasons of excellence in mechanical strength and dimensional accuracy of molded intermediate product, convenience in mass production, etc.
While the injection molding is useful in case the product is complicated in shape and to be produced in a large quantity, it actually encounters a problem that many of intermediate products are liable to cause breakage, deformation, etc. due to long time heating at the time of dewaxing, resulting in a strict limitation or restriction on the shape and thickness of the product.
For the purpose of solving the problem, a ceramic composition for injection molding which can be easily dewaxed even when the product is thick-walled has been proposed by Japanese provisional patent publication Nos. 54-9516 and 55-23097. It has further been proposed by Japanese provisional patent publication No. 57-17468 a dewaxing process for removing binder from the product by heating while holding the same in pressurized gas of pressure higher than an atomospheric pressure. The above ceramic composition and the dewaxing process are effective for solving the above problem to some extent but not satisfactorily.
For solving the above problem, there has also been proposed by Japanese provisional patent publication No. 60-11276 a method of producing a ceramic rotor. The method consists of producing the disk portion of a rotor by injection molding, the thickness of which disk portion is so determined as to enable the disk portion to be dewaxed easily, producing the shaft portion by cold isostatic pressing so as to be dewaxed easily, assembling the disk portion and the shaft portion together by cold isostatic pressing and sintering the assembled disk portion and shaft portion into a single sintered body. In this instance, the disk portion and the shaft portion effect different expansions and contractions during sintering. Due to this, tension occurs in the disk portion at the time of sintering and remains thereat as residual stress after the sintering. In addition to the residual stress, the rotor, particularly the disk portion thereof is subjected to centrifugal stress at the time of rotation. Since the allowable stress of the rotor is limited by the tension strength of the material forming the rotor, the residual stress in the disk portion causes an increased limitation or restriction on the design of product.